Causes

A blocked milk duct. If a breast doesn't completely empty at feedings, one of your milk ducts can become clogged, causing milk to back up, which leads to breast infection.

Bacteria entering your breast. Bacteria from your skin's surface and baby's mouth can enter the milk ducts through a break or crack in the skin of your nipple or through a milk duct opening. Bacteria can multiply, leading to infection. These germs aren't harmful to your baby — everyone has them. They just don't belong in your breast tissues.

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